JPS61253584A - Picture processor - Google Patents

Abstract

PURPOSE:To control satisfactorily the deterioration of a half-tone picture by detecting the amplitude of density variance in a partial area of an original picture and selecting a dither matrix for application according to the value of the amplitude. CONSTITUTION:The picture signals PX are supplied in the quantity equal to four lines and the picture elements corresponding to a dither matrix are extracted by a matrix register 4. Then an addition circuit 5 calculates the key value data DM and outputs it to an FIFO buffer 6. The key value data DM corresponding to the group area is extracted with a matrix register 7 for each input of signals PX equivalent to 12 lines. A CPU 10 detects the discriminates the amplitude of the data DM and also stores the selection data SD to a memory 11. Then the CPU 10 outputs the data SD to a selector 23 in the form of the control signal CS. The threshold value signal of a Bayer type dither matrix generator 24 or a spiral dither matrix generator 25 is selected as the threshold value signal TL according to the density of the group area including the signals PX.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to an image processing device for expressing a halftone image using a binary image signal.

[Prior art] Generally, the size of the dots that make up one image is constant.

In order to express a halftone image (for example, an image with shading or a color image) using a recording device in which each dot can take only a binary state of black and white, the following method is used.

That is, first, the image is decomposed into pixels corresponding to the above-mentioned dot size, and the density of each pixel is read as a multi-value. This multilevel density signal is then binarized using a threshold based on a systematic dither matrix of a predetermined size to form a pseudo halftone signal.

A halftone image can be obtained by outputting the halftone signal to a recording device such as the one described above to record an image.

Now, the systematic dither matrix described above pseudo-halftones an image by converting the image of a predetermined small area into a density of black pixels corresponding to the density of the image, and the signal arrangement Depending on the configuration, a Bayer dither matrix as shown in FIG. 2, a spiral dither matrix as shown in FIG. 3, and a fourth dither matrix as shown in FIG.
It is classified as a halftone dither matrix as shown in the figure. Note that the size of the items shown in these figures is 4×
The number of gradations increases as the size increases.

Each of these organized dither matrices has the following characteristics.

Since the Bayer dither matrix is a 1-pixel distributed type, it is less likely to cause moiré caused by the periodicity of the original image, the periodicity of image sampling, and the periodicity of processing, but In areas where the density changes, where the density is visually perceived as continuously changing, a stepwise density change called a so-called false edge occurs, causing deterioration of one image.

For example, in a region as shown in FIG. 5(a) where it is visually perceived that the concentration is changing continuously over a wide area,
Even if the image is gradated as shown in FIG. 6(b), there is no problem because it is visually perceived as a density change similar to that of the original image.

On the other hand, if a region of the original image where density fluctuations occur at a low frequency has a partial rapid density fluctuation as shown in FIG. 6(a), in this original image, Visually, a sudden change in density in that part is not perceived, but a continuous change in density is perceived as in the surrounding area. However, according to the Bayer dither matrix, the gradation is performed in response to this density variation, as shown in FIG. Since it is reliably visible, such a part is sensed as a boundary (edge). Such edges are called false edges.

In addition, although the spiral dither matrix hardly produces the above-mentioned false edges, since it is a dot-concentrated type, the periodicity in the processed image is noticeable. The fluctuations are superimposed, and the influence of the moiré image is large.

Further, the halftone dither matrix has characteristics intermediate between the Bayer dither matrix and the spiral dither matrix described above.

As described above, due to the characteristics of each dither matrix, when the entire area is gradated using one dither matrix, the original image partially deteriorates.

[Objective] The present invention has been made to solve the above-mentioned disadvantages of the prior art, and an object of the present invention is to provide an image processing device that can sufficiently suppress deterioration of halftone images.

[Configuration] In order to achieve this objective, the present invention detects the amplitude of density fluctuation in a partial region of the original image, and selects the optimal dither matrix to be applied to the region according to the magnitude of the amplitude. There is.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 shows an example of an image processing apparatus according to an embodiment of the present invention. In this embodiment, the size of the dither matrix is set to 4×4, and the Bayer type dither matrix and the full volume type dither matrix are used by switching.

In the figure, both signals PX having a predetermined number of bits (for example, 4 bits) output from an image reading device (not shown) for each line are sequentially delayed by l lines by line buffers 1 and 2. It is output to 4. In addition, output delayed image signals DPI, DP2 . DP3 is also output to matrix register 4.

The matrix register 4 is for extracting pixels of a small area corresponding to the dither matrix, and the extracted data is summed by an adder circuit 5, and the timing is adjusted as representative value data DH indicating the density tendency of this small area. The data is stored in a FIFO (first-in-first-out) buffer 6.

The representative value data DH stored in the FIFO buffer 6 is
The signals are sequentially read out at predetermined timing and added to the matrix register 7, and are delayed by the line buffers 8 and 9. Delay representative value data DMI and DM2 of the line buffers 8 and 9 are also added to the matrix register 7.

The matrix register 7 extracts a group area formed by combining a plurality of small areas (9 in this case) adjacent to each other in the main scanning direction and the sub-scanning direction, and the extracted data of this matrix register 7 is as follows. The data is input to a CPU (central processing unit) 10.

The CPU10 determines the maximum value and minimum value of the extracted data input from the matrix register 7, and calculates the difference between them, that is,
The amplitude of the representative value of the small area in the group area is detected, and the amplitude is compared with a predetermined value.

If this amplitude is greater than a predetermined value, select data SO of data rlJ indicates that a Bayer dither matrix is selected in the group region.

Data "0" is stored in the memory 11 in association with the scanning address of the read image corresponding to the group area, and if the amplitude is smaller than a predetermined value, the data "0" is stored to indicate that the spiral dither matrix should be selected in the group area. selection data S
D is stored in the memory 11 in association with the scan address of the read document corresponding to the group area.

On the other hand, the delayed image signal DP3 of the line buffer 3 receives the input data for a time corresponding to the sum of the operating times of the adder circuit 5, the FIFO buffer 6, and the matrix register 7 and the time required for the analysis process in the CPU10.
The output of the delay buffer 20 is delayed through the delay buffer 20, and the timing at which it is applied to the FIFO buffer 21 and output to the comparison input terminal of the comparison circuit 22 is adjusted.

The comparison circuit 22 receives a threshold signal TL via a selector 23.
is applied to its reference signal input terminal, and executes the binarization process of the image signal. The binary signal DD obtained by the comparator circuit 22 is subjected to timing adjustment via the timing adjustment buffer 16, and is output to the next stage device.

The selector 23 is applied with threshold signals TLI and TL2 generated from a Bayer dither matrix generator 24 and a spiral dither matrix generator 25, respectively, and a control signal C8 applied from the CPU10 via the output circuit 12. When the control signal C8 is "1", the threshold signal TLI is selected, and when the control signal C8 is "0", the threshold signal TL2 is selected and output to the comparison circuit 22 as the threshold signal TL.

Furthermore, the Bayer dither matrix generator 24 and the spiral dither matrix generator 25 are synchronized with the dither clock CD applied from the clock generator 27.
The elements of the corresponding row are generated repeatedly for each line. In other words, when the operation starts, the four elements of the first line are repeatedly generated, and when the line is finished, the four elements of the second line are repeatedly generated, and each time the line is finished, the next one is sequentially generated. The elements of the row are repeatedly generated, and when this is repeated up to the fourth row, the signal generation is repeated again by returning to the elements of the first row.

When the clock generator 27 receives a start signal ST from the CPU10 via the output circuit 12, the clock generator 27 outputs the line buffer 1.
, Shift clock SPI that drives line buffers 8 and 9, Shift clock SP that drives line buffers 8 and 92) FIFO
Delay clock DCI and FIF that drive buffer 6
A delay clock DC2 that drives the O-buffer 21, a dither clock CD that drives the Bayer dither matrix generator @24, and the spiral dither matrix generator 25 are generated and output to the corresponding elements, respectively.

Note that the line buffers 1, 2.3 are driven in synchronization with the pixel clock of the image signal Px, the FIFO buffer 6 is driven in synchronization with the latch timing of the addition result in the adder circuit 5, and the line buffers 8, 9 are driven in synchronization with the pixel clock of the image signal Px. buffer 6
The Bayer dither matrix generator 24, the spiral dither matrix generator 25, and the FIFO buffer 21 are driven in synchronization with the output timing.

With the above configuration, when the input of the image signal Px is started.

CPU10 outputs a start signal ST to start the clock generator 27 and output each timing signal. As a result, when four lines of the image signal Px are input and the matrix register 4 extracts pixels corresponding to the dither matrix, the adder circuit 5 calculates the representative value data DM and
Output to IFO buffer 6. That is, the adder circuit 5 outputs the representative value data DM every time it receives the pixel signal px of four pixels at a rate of one line for every four lines of the pixel signal Px.

Therefore, every time 12 lines of the image signal Px are input to the matrix register 7, the representative value data DM corresponding to the group area is extracted. Also, at the timing of CPt1
1G inputs the extracted data of the matrix register 7. Note that this extracted data is valid until the next representative value data DM is output from the FIFO buffer 6, and cputo completes the input of the extracted data within that time.

In this way, the CPU 10 inputs the representative value data DM corresponding to the group area at a ratio of 1 line to 12 lines, so that the above-mentioned amplitude detection process, amplitude discrimination process, and selection into the memory 11 of the representative value data DM are performed. Storage processing of data SD is executed in the first half of the data input interval.

Also, in the latter half of the data input interval, the comparison circuit 2
The threshold signal TL corresponding to the image signal Px added to 2 is
The threshold value selection process selected by the selector 23 is executed.

That is, the CPU 10 determines the scan address in the read image of the image signal Px output from the F'o buffer 721, and stores the memory 11 corresponding to the scan address.
The stored contents (ie, selection data SD) are read out.

Then, the -2 selection data SD is outputted to the selector 23 via the output circuit 12 as a control signal C8.

As a result, either the threshold signal TLI output from the Bayer type dither matrix generator 24 or the threshold value signal TL2 output from the spiral dither matrix generator 25 is generated depending on the density state of the group area including the one-image signal PX. is selected as the threshold signal TL corresponding to the image signal px. Further, the selection state of the threshold signal may be switched at the timing of switching the group areas.

As mentioned above, cputo can perform halftone processing on both signals Px in real time.

It is possible to form a halftone image that corresponds to the tendency of local density fluctuations in the image, and it is possible to prevent extra time from being taken for this purpose.

Incidentally, in the embodiment described above, the amplitude of the representative value data DH in the first group region is calculated as an element for determining the tendency of concentration fluctuation within the group region, but such an element is not limited to this.

For example, MTF (Modulationτtransfer F) is used to correct image deterioration due to reading system characteristics.
function) correction processing can be applied. That is, in this TF correction process, the density of the pixel of interest is
If the density of the surrounding pixels varies significantly compared to the density of surrounding pixels, the tendency of the density variation is further increased.
This corrects blur in images.

In this case, the index data ID that shows the tendency of density fluctuation in the group region is calculated using the linear equation.

IDsu,lpAwa-q,:1bmaIwhere, A indicates a representative value of the small region of interest, and b■n indicates a representative value of a plurality of small regions adjacent to the small region of interest.

Moreover, when the representative values of each of the above small regions are the same, PeQ is
This is a coefficient set so that the value of index data In becomes 0. Note that the small region of interest and the adjacent small region do not need to be in the same group region.

Then, the sum of this index data ID in each group region is formed, and if the sum is greater than a predetermined value, a Bayer dither matrix is applied to the group region, and if the sum is smaller than a predetermined value, etc. A spiral dither matrix is selected in each region.

Furthermore, in the embodiment described above, the dither matrix to be applied is selected for each group region, but depending on the linear density of the image signal Px, this group region may become quite small, and in such a case, the image There is a risk of deteriorating quality. Therefore, in such a case, dither matrices are switched on the condition that the same dither matrix is selected in a plurality of consecutive group areas, thereby preventing frequent switching of dither matrices.

Note that the size of the dither matrix, the shape of the small region, and the shape of the group region are not limited to those described above. Further, the representative value may be set to the average value of the image signal level of a small area, and furthermore, a halftone dither matrix may be used instead of a spiral dither matrix.

[Effects] As explained above, according to the present invention, the amplitude of density fluctuation in a partial region of an original image is detected, and the optimal dither matrix to be applied to the region is selected based on the magnitude of the amplitude. Therefore, it is possible to form a halftone image with extremely high image quality.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an image processing device according to an embodiment of the present invention, FIG. 2 is a signal arrangement diagram illustrating a Bayer dither matrix, and FIG. 3 is a signal arrangement diagram illustrating a spiral dither matrix. . Figure 4 is a signal arrangement diagram illustrating a halftone dither matrix, Figure 5 (a) is a graph diagram showing an example of density changes in an original image, and Figure 5 (b) is shown in Figure 5 (a). A graph diagram showing a gradation processing state corresponding to density changes. Fig. 6(a) is a graph showing another example of the density change of the original image, and Fig. 6(b) is a graph showing the gradation processing state corresponding to the density change shown in Fig. 6(a). It is a diagram. 1.2, 3, 8, 9... line buffer, 4.7...
・Matlin 7x register, 5...addition circuit,
6, 21...FIFO buffer, 10...cpu
(central processing unit), 11... memory. 12... Output circuit, 20... Delay buffer, 22
... Comparison circuit, 23 ... Selector, 24 ... Bayer dither matrix, 25 ... Spiral dither matrix, 26 ... Timing adjustment buffer, 27
...Clock generator. Figure 2 Figure 3 Figure 4 Figure 5 (a) (b) Figure 6 (a) (b) Procedural amendment band (July 15, 1985)

Claims (2)

[Claims] (1) A read image is decomposed into pixels of a predetermined size to form a multi-value density signal corresponding to the density of each pixel, and this multi-value density signal is binarized using a threshold based on a dither matrix of a predetermined size, and halftone In an image processing device that forms an image, a dither matrix generating means generates two dither matrices of different sizes, and corresponds to the level of the multilevel density signal of a small area consisting of a plurality of pixels corresponding to the dither matrix. representative value calculation means for calculating the representative value for each small region; representative value amplitude calculation means for calculating the amplitude of the representative value in a group region consisting of a plurality of adjacent small regions; and the representative value calculation means. an amplitude discriminating means for discriminating the amplitude state of the representative value in the group region from the output of the above; a memory means for storing the output of the amplitude discriminating means for each of the group regions; An image processing apparatus comprising a dither matrix selection means for selecting one of the two dither matrices for each group area, and forming a threshold based on the dither matrix selected by the dither matrix selection means. (2) The image processing apparatus according to claim 1, wherein the dither matrix generating means generates a dot-distributed dither matrix and a dot-concentrated dither matrix.